1. Field of the Invention
The present invention relates to a projection display, and more particularly, to a projection display adopting a compact light source such as a light emitting diode.
2. Description of the Related Art
FIG. 1 shows the structure of a conventional projection display. Referring to FIG. 1, the conventional projection display includes liquid crystal display (LCD) panels 20R, 20G, and 20B which are optical modulators, an illumination unit 10 which irradiates light onto the LCD panels 20R, 20G and 20B, and a projection lens 40 which magnifies and projects a modulated image.
The LCD panels 20R, 20G, and 20B modulate red (R), green (G), and blue (B) beams, respectively, according to respective image data so as to display a color image. Reference numeral 30 denotes a synthesizing prism which combines the modulated R, G, and B beams into one beam and then irradiates the combined beam onto the projection lens 40.
The illumination unit 10 includes a light source 1, an integrator 3, a condenser lens 4, a plurality of mirrors 5R, 5G, and 5B, and a plurality of relay lenses 7 and 8.
The light source 1 may be a metal halide lamp or a super-high voltage mercury lamp, and is located at a focal point of a reflective mirror 2 with a parabolic surface. The integrator 3 is used to irradiate a uniform beam onto the LCD panels 20R, 20G and 20B and is generally made of two fly-eye lenses in which micro-lenses are two-dimensionally arrayed. A light beam, which has passed through the integrator 3, is condensed by the condenser lens 4. The mirrors 5R, 5G, and 5B are selective reflector mirrors which reflect the R, G and B beams, respectively, and transmit other color beams. A light beam is split into the R, G, and B beams via the mirrors 5R, 5G, and 5B, respectively, and then the R, G, and B beams are incident on the LCD panels 20R, 20G, and 20B, respectively, through the relay lenses 7 and 8. The LCD panels 20R, 20G, and 20B modulate the R, G, and B beams, respectively, so as to output R, G, and B color images. The synthesizing (chroic) prism 30 combines the R, G, and B beams output from the LCD panels 20R, 20G, and 20B into one beam, and then the projection lens 40 magnifies and projects the combined beam.
However, in such a conventional projection display, a lamp is used as a light source to illuminate optical modulators and has a short life span. Therefore, when the conventional projection display is used in homes, the lamp should be frequently replaced with a new one. Also, the light source is large in size. In order to solve these problems, studies on the use of compact light sources, such as a light emitting diode (LED) with a relatively long life span, etc., are in progress. Japanese Patent Publication No. JP 2001-42431 discloses a projection device using an LED.
In order to increase an amount of light to be effectively projected by the projection lens 40, the conventional projection display requires secondary optics to collimate a light beam radiating from the LED before irradiating the light beam onto the optical modulators. As a result, the additional use of the secondary optics makes an illumination system of the conventional projection display complicated, and increases costs of manufacturing the illumination system. In general, an LED emits a smaller amount of light than a metal halide lamp or a super-high voltage mercury lamp. Thus, the conventional projection display uses an array of LEDs as a light source. In this case, secondary optics is necessary. However, since the secondary optics has to be lenses, light condensing efficiency deteriorates.